1. Field of the Invention
The present invention relates to image forming apparatuses.
2. Description of the Related Art
In many image forming apparatuses that form images by electronic photography process, such as printer, copier, facsimile, and multi-function peripheral thereof, a toner image is developed on a photoconductor drum, and the toner image is transferred onto an intermediate transfer belt, and then transferred from the intermediate transfer belt to a paper sheet, and finally the toner image is fixed on the paper sheet.
Such image forming apparatuses perform toner density correction and color registration correction when required or periodically.
In general, in a process of the density correction, a toner pattern (i.e. plural toner patches) as reference is developed and transferred to the intermediate transfer belt, toner densities of the toner pattern on the intermediate transfer belt is determined using an optical sensor, and the density correction is performed on the basis of the toner densities. Further, in case of a color image forming apparatus, positions of the toner pattern on the intermediate transfer belt are determined, and the color registration correction is performed on the basis of the positions.
Furthermore, there is another technique to determine toner densities on the photoconductor drum.
In a sensor for measuring the toner densities, a beam splitter separates a light beam emitted from a light emitting diode into a predetermined polarized component (either P-polarized component or S-polarized component), and the predetermined polarized component enters the toner pattern on the intermediate transfer belt. Reflection light from the toner pattern is separated into a specular reflection component and a diffuse reflection component by a beam splitter, and these components are detected by photo detectors. Finally, the toner density is determined from intensities of the specular reflection component and the diffuse reflection component. If the incident light to the toner pattern is P-polarized, then the specular reflection component is P-polarized, for instance.
Further, another image forming apparatus uses a low pass filter to reduce noise due to a stain or a scratch on a surface in toner density measurement of a low density range.
However, since toner absorbs light, it is difficult to precisely measure toner densities of all the toner patches from low density range to high density range using the aforementioned reflection-type sensor.
For example, when a toner density is calculated on the basis of a difference between measurement values of the specular reflection component and the diffuse reflection component, the difference is small under some measurement conditions of the sensor, and it causes low measurement precision. In particular, black toner strongly absorbs light, and it may cause very low measurement precision of black toner.